The shape of the lesson
By the end of the lesson, learners can
- Peppered moths on the bark: The sim starts with 40 moths on a tree trunk, 10% of them dark.
- Antibiotic resistance: Extended.
- Adaptation over generations: Extended.
- Variation: genetic variation within the population (from mutation).
- Overproduction and struggle: many offspring; competition, predation, disease.
- Selection: better adapted individuals are more likely to survive and reproduce, and pass on their alleles.
- Never say the environment causes the useful mutation or that individuals change to adapt.
One picture every idea returns to
Natural selection: there is genetic variation within a population; organisms produce many offspring; there is a struggle for survival, including competition for resources; individuals that are better adapted to the environment have a greater chance of surviving and reproducing; they pass on their alleles to the next generation. Extended: over many generations this makes populations more suited to their environment, a process called adaptation.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Survival of the better adapted.. Birds eat the moths they can see. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Peppered moths on the bark | The sim starts with 40 moths on a tree trunk, 10% of them dark. Project tab 5 of the lab (Natural selection in peppered moths) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Antibiotic resistance | Extended. Project tab 6 of the lab (Antibiotic resistant bacteria) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Adaptation over generations | Extended. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 24 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Adaptation over generations, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The moths do not turn dark because the bark is sooty, and they do not choose to change. | The dark allele was already in the population; the environment only changes which moths survive to breed. |
| The antibiotic does not cause the resistance mutation. | The mutation happens at random; the antibiotic only selects the bacteria that already carry it. |
| Individuals do not adapt during their lifetime in this sense. | A single moth cannot change its wing colour; only the population changes, as alleles are passed on. |
Natural selection: worksheet
6 questions, 24 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the process of natural selection.
Scientists counted peppered moths in a polluted woodland, where the bark was dark with soot, over several years after clean-air laws.
| Year | 1970 | 1980 | 1990 | 2000 |
|---|---|---|---|---|
| Dark moths / % | 90 | 70 | 35 | 10 |
(a) Describe the trend. (b) Use natural selection to explain it.
Giraffes with longer necks can reach leaves high in trees. Use natural selection to explain how the average neck length in a giraffe population could increase over many generations.
Extended. Describe how a strain of bacteria resistant to an antibiotic develops.
Extended. Suggest three ways in which doctors and patients can reduce the spread of antibiotic resistant bacteria.
Extended. Distinguish between an adaptive feature and adaptation.
Answer key and marking notes
- There is genetic variation within a population.
- Organisms produce many offspring, more than can survive.
- There is a struggle for survival, including competition for resources such as food.
- Individuals that are better adapted to the environment are more likely to survive and reproduce.
- They pass on their alleles to the next generation, so these alleles become more common.
- (a) The percentage of dark moths fell from 90% to 10% over 30 years.
- (b) Clean air let lichens grow back, so the bark became paler.
- Dark moths were now more visible to birds and more were eaten; pale moths were camouflaged and more survived to reproduce.
- Pale moths passed on their alleles, so the pale form became more common each generation.
- Neck length varies in the population because of genetic variation.
- When food is short there is competition; giraffes with longer necks reach more leaves.
- They are more likely to survive and reproduce.
- They pass on the alleles for long necks, so the average neck length increases over many generations.
- A random mutation gives one bacterium an allele for resistance to the antibiotic.
- When the antibiotic is used, non-resistant bacteria are killed.
- The resistant bacteria survive; this is natural selection.
- They reproduce rapidly with less competition and pass on the resistance allele.
- Over many generations most of the population is resistant: a resistant strain.
- Only prescribe or use antibiotics when they are really needed (not for viral infections such as colds).
- Patients should complete the full course, so that all the bacteria are killed.
- Good hygiene in hospitals, such as hand washing, stops resistant bacteria spreading between patients. (Also accept: avoid routine use in farm animals.)
- An adaptive feature is an inherited feature of an individual that helps it survive and reproduce in its environment.
- Adaptation is a process, resulting from natural selection,
- by which a population becomes more suited to its environment over many generations.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement is a step in natural selection?
- AIndividuals better adapted to the environment are more likely to reproduce
- BIndividuals change their features during life to suit the environment
- CHumans choose which individuals are allowed to breed
- DAll the offspring produced survive to become adults
A frog lays 2000 eggs but only about 10 frogs from them survive to breed. Which part of natural selection does this show?
- AMany offspring are produced and there is a struggle for survival
- BGenetic variation is produced by mutation
- CBetter adapted frogs pass on their alleles
- DHumans select frogs with useful features
Rabbits on an island vary in how fast they can run. Foxes arrive on the island. What is most likely to happen over many generations?
- AFaster rabbits survive and breed more, so fast running becomes more common
- BEvery rabbit learns to run faster and passes this on to its young
- CThe foxes cause mutations that make the rabbits run faster
- DThe rabbits stop varying, so all of them run at the same speed
In natural selection, what is passed from the survivors to the next generation?
- AAlleles that helped them survive
- BFeatures acquired during their life
- CThe resources they won in competition
- DImmunity they gained from vaccines
In a clean woodland 5% of peppered moths are dark. After a factory makes the bark sooty, the percentage of dark moths rises to 80% over 50 years. Which explanation is correct?
- ADark moths were better camouflaged, so fewer were eaten and more reproduced
- BSoot settled on the pale moths and turned them dark
- CThe soot caused mutations in pale moths, turning them dark
- DPale moths chose to move away to cleaner woodland
Extended: the graph shows infections caused by a resistant strain of bacteria in a hospital after an antibiotic was introduced. What is the best explanation for the trend?
- AThe antibiotic killed non-resistant bacteria, so resistant ones survived and multiplied
- BThe antibiotic caused resistance mutations in more bacteria each year
- CPatients became resistant to the antibiotic, so it stopped working
- DResistant bacteria were added to the hospital by the doctors
Extended: use the graph of infections caused by the resistant strain. By how many percentage points did the share rise between year 4 and year 8?
- A30
- B40
- C4
- D38
Extended: which statement correctly describes adaptation?
- AA process by which a population becomes more suited to its environment over many generations
- BA change in one organism during its lifetime that helps it survive
- CAn inherited feature of an individual that helps it survive and reproduce
- DThe deliberate breeding of animals with features useful to humans
Quiz key
Better adapted individuals have a greater chance of surviving and reproducing, and pass on their alleles. Individuals do not change their inherited features to suit the environment. Humans choosing the parents is artificial selection. Natural selection depends on more offspring being produced than can survive, so not all of them reach adulthood.
Producing far more offspring than survive shows overproduction, which leads to a struggle for survival through predation, disease and competition. The numbers say nothing about mutation or which alleles are passed on. Humans are not involved, so it is not artificial selection.
Faster rabbits are more likely to escape foxes, survive and reproduce, passing on alleles for fast running, so this feature becomes more common. Speed gained by practice is not inherited. Predators do not cause mutations; variation already existed. Variation remains in the population; selection changes how common each allele is.
Survivors pass on their alleles, in their gametes, to their offspring. Acquired features such as scars or trained muscles are not inherited. Resources such as food or territory are not genetic. Immunity from vaccines is acquired during life and is not passed on in the genes.
On sooty bark dark moths are camouflaged from birds, so more survive to reproduce and pass on the dark allele; over many generations the dark form becomes common. Soot on a moth's wings is not inherited. The dark allele was already present, so soot did not have to cause it; selection changed its frequency. Moths do not choose to change the population; birds remove the visible ones.
The antibiotic is a selection pressure: it kills the non-resistant bacteria, and the resistant ones survive, reproduce and pass on the resistance allele, so their share rises from 2% to 48% over 10 years. The mutation is random and is not caused by the antibiotic. It is the bacteria, not the patients, that become resistant. Nothing suggests resistant bacteria were deliberately added.
At year 4 the share is 10% and at year 8 it is 40%, so it rose by 40 − 10 = 30 percentage points. 40 is the year 8 value alone. 4 is the year 2 value, read from the wrong point. 38 is 40 − 2, using the year 0 value instead of year 4.
Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. Changes within one lifetime are not inherited. An inherited feature of an individual is an adaptive feature, the result of adaptation rather than the process. Deliberate breeding by humans is selective breeding.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.